SQL & PL/SQL

Hi all,

I am having table like this

Create table Request
( x number, y varchar2(20), a number, b varchar2(3),
Constraint pk_Request PRIMARY KEY (x, y,a) )

Now, i want to
1. change the compoiste primary key to only x and y (First having 3 fields, now
only two fields)
2. If i want to add one more field (i.e. composite PK contains x,y,a,b)

Thanks in advance,
Pal